1. Field of Invention
The invention relates to the method for printing apparatus halftone screen calibration. More specifically the invention is used in the halftone screen creation process.
2. Related Art
While printing, the printer usually needs to perform the color conversion from RGB to CMYK color space. Typically, the 3-bytes RGB electronic data are converted to 4-bytes CMYK data, wherein R, G, B, C, M, Y and K are respectively represented by one byte with a value between 0-255. During color rendering process, the CMYK values of the original image are compared with the corresponding values in the screen table to determine whether each dot has to be “printed”.
FIG. 1 is a schematic view illustrating a halftone screen table. It is formed of a 16×16 matrix having 256 grid units for filling 0-255 gray levels (although the figure only shows values of 255-224, the values of other grid units may be readily inferred). The gray level in each grid unit of the screen table represents a reference value that will be compared with the raw data to determine whether the corresponding dot should be printed.
How to print the dot depending on the settings of the command program (such as PCL, PS3, etc.). For example, when the raw data is greater (smaller) than the value of the screen table, then a dot will be printed.
FIG. 2 depicts a case where dot will be printed when the value of the screen table is greater than the raw data. Suppose we want to print a square area with gray level 247. By comparing the screen table (FIG. 1) with 247, we can get the dots printed like FIG. 2.
In addition to monochrome printing, a color printing system can apply the same concept to adjust CMYK screen tables respectively.
Each printing apparatus may have different characteristics. Therefore the halftone screen generated for one apparatus may behave differently due to those characteristics, such as dot gain, toner characteristic, etc. A screen calibration consequently is used to perform on each model of printer engine to obtain an optimal image/data output.
U.S. Pat. No. 5,953,498 discloses a nonlinear calibration of output devices to solve the problems relating to image color intensity. In its calibration method, 8-bit gray level data of the image are converted to 16-bit gray level data in which compensation information, used to perform data calibration, is further added. However, this calibration method usually needs the installation of a permanent program or in other cases the installation of additional equipment, which is burdensome and practically inconvenient.